[libc] Skip fuzzer as well if its dependent entrypoints are skipped.

libc/cmake/modules/LLVMLibCTestRules.cmake

message(FATAL_ERROR "'add_libc_fuzzer' target requires a SRCS list of .cpp "
                         "files.")
   endif()
   if(NOT LIBC_FUZZER_DEPENDS)
     message(FATAL_ERROR "'add_libc_fuzzer' target requires a DEPENDS list of "
                         "'add_entrypoint_object' targets.")
   endif()
 
   get_fq_target_name(${target_name} fq_target_name)
+  get_fq_deps_list(fq_deps_list ${LIBC_FUZZER_DEPENDS})
+  get_object_files_for_test(
+      link_object_files skipped_entrypoints_list ${fq_deps_list})
+  if(skipped_entrypoints_list)
+    set(msg "Skipping fuzzer target ${fq_target_name} as it has missing deps: "
+            "${skipped_entrypoints_list}.")
+    message(STATUS ${msg})
+    add_custom_target(${fq_target_name})
+
+    # A post build custom command is used to avoid running the command always.
+    add_custom_command(
+      TARGET ${fq_target_name}
+      POST_BUILD
+      COMMAND ${CMAKE_COMMAND} -E echo ${msg}
+    )
+    return()
+  endif()
+
   add_executable(
     ${fq_target_name}
     EXCLUDE_FROM_ALL
     ${LIBC_FUZZER_SRCS}
     ${LIBC_FUZZER_HDRS}
   )
   target_include_directories(
     ${fq_target_name}
     PRIVATE
       ${LIBC_SOURCE_DIR}
       ${LIBC_BUILD_DIR}
       ${LIBC_BUILD_DIR}/include
   )
 
-  get_fq_deps_list(fq_deps_list ${LIBC_FUZZER_DEPENDS})
-  get_object_files_for_test(link_object_files ${fq_deps_list})
   target_link_libraries(${fq_target_name} PRIVATE ${link_object_files})
 
   set_target_properties(${fq_target_name}
       PROPERTIES RUNTIME_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
 
   add_dependencies(
     ${fq_target_name}
     ${fq_deps_list}
   )
   add_dependencies(libc-fuzzer ${fq_target_name})
 endfunction(add_libc_fuzzer)